New explanation for bends in streams

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Roelof Kleis

Bends in peatland streams such as the Drentsche Aa are almost right-angled. PhD candidate Jasper Candel of the Soil Geography and Landscape chair group investigated how these bends arose and has come up with an elegant theory.

Many streams and brooks in the Netherlands were straightened out and channelled in the second half of the 20th century so that surplus water could be rapidly removed. In the past couple of decades, many of those streams have had their bends restored for reasons of water quality and ecological management. Old maps often serve as a guide. But how did the streams acquire those bends? To answer this question, Candel went far back in time.

The PhD candidate focused on the Drentsche Aa, one of the few streams in the Netherlands that escaped the channelling process. The stream meanders through the peaty landscape in a pattern rather like a square wave. The usual explanation for the formation of bends in sandy soil is that the fast-moving water erodes the soil away. But that explanation doesn’t work for peatland streams. Candel: ‘Peat is a tough material, and the water in the Drentsche Aa is slow-moving. As a result, the stream hardly changes course at all.’

Candel started drilling to get a picture of the old stream sediments. He was able to use the drilling, soil radar, carbon-14 dating and optically stimulated luminescence to reconstruct the stream’s course and the age of the sediments. It turns out that the flat area through which the Drentsche Aa runs has changed dramatically since the last Ice Age, around 12,000 years ago.

According to Candel, the Drentsche Aa started out in one of the deep valleys left behind by the Ice Age. Those valleys gradually filled up with peat due to the warmer, wetter climate. Candel found peat down to a depth of eight metres. ‘The stream rose up as the peat accumulated. All that time, the stream followed the contours of the hillside. That sand eroded more easily than the peat. So the peat was actually pushing the stream upwards against the hillside.’ One part of the stream ‘walked’ along one hillside, another part ‘walked’ along the opposite hillside and at certain points, the stream crossed over from one side to the other. Which is why the Drentsche Aa has almost right-angled bends.

Candel says that as the layer of peat grew thicker, successive bends were pushed increasingly far apart — until the peat and the stream rose above the level of the hilltops. The stream has not changed course since then.